Conventional signal filters often utilize highpass and lowpass filter elements based on RC filter designs. The pole of a typical RC filter is located at a frequency of 1/RC. Unfortunately, it is generally difficult to accurately set the resistance and capacitance in a cost-effective manner while producing large quantities of circuits. Improved signal filters are constructed which utilize switched capacitor filters which are advantageous because they provide a very precise pole for low pass filter and high pass filters. These poles are based on the clock frequency and the on-chip capacitor ratio. The pole of a typical RC filter resides at a frequency of 1/RC. The pole of a switched capacitor filter occurs at a frequency equal to the clock frequency times the ratio of two capacitors, f*(C.sub.2 /C.sub.1). Because the pole of a switched capacitor filter is determined by a ratio of capacitances, positioning of the pole is process independent. The oxide thickness of an integrated circuit is substantially constant throughout the integrated circuit chip so that the oxide thickness is the same for both capacitors.
Although the location of a filter pole is process independent, the location is sensitive to the clock frequency of a timing signal that controls the switch of the switched capacitor filter. Conventional switched capacitor filters utilize a crystal oscillator to precisely set positions of the poles of the plurality of filters. The poles are placed precisely in selected positions by selecting the relationships of the capacitance of the two capacitors in a filter and by the timing frequency of the circuit. Unfortunately, crystal oscillators are very expensive relative to other oscillators. Furthermore, a crystal oscillator generates a fixed frequency so that variable-frequency filter designs are not simply constructed.
What is needed is an inexpensive, simple and adjustable filter circuit.